A number of aspects concerned with the biosynthesis of ribosomes which include the post-transcriptional and post-translational modifications of rRNA and ribosomal proteins will be examined. Emphasis is placed upon the determination of the RNA and protein compositions of the various intermediates obtained from mutants defective in ribosome assembly. The above information shall provide the basis for studying the post-transcriptional modification of ribosomal RNA (such as methylation and the formation of ribothymine and pseudouracil). The function of the modification reactions can also be examined by in vitro reconstitution of particles deficient in one (or a few) modifications. For the post-translational methylation of ribosomal proteins, experiments are described to determine the proteins that are methylated. The nature and stoichiometry of the methylated amino acid(s) in each of the methylated proteins will be determined, and the function of the methylation of ribosomal proteins will be investigated by in vitro reconstitution of particles deficient in specific methylation.